While indoor air quality and ventilation have been major focuses of the HVAC industry for a long time, many misconceptions remain about what these applications require in terms of equipment and design. Today’s post will parse some of the most common questions about DOAS and comfort cooling equipment.
Purpose of DOAS
A dedicated outside air system (DOAS) or energy recovery unit (ERU) conditions air from the outdoors and delivers it to a space. It is essentially a dehumidifier in that its primary purpose is to remove moisture from the air, so when that air enters the space it is not adding additional load. This contrasts with comfort cooling equipment whose purpose is to maintain a consistent temperature in the space.
How
In order to achieve a cooling effect, a comfort cooling device must provide air to the space at a temperature much lower than the set point for that space. A DOAS or ERU, for its part, is not concerned with the temperature in the space. It monitors the temperature and humidity of the outdoor air and gauges how much capacity it needs to remove the moisture. The DOAS/ERU then discharges dry, room neutral air (roughly 70°-72°F and 30-40% RH).
When you need comfort cooling
A comfort cooling device operates only when the temperature in a space rises above set point. An outdoor air system, however, will typically operate during all occupied hours. ASHRAE 62.1 requires outdoor air be brought into in buildings at all times, so these machines rarely rest.
What is a DOAS unit?
Outdoor air equipment typically discharges air off of its cooling coil at 50-55°. The low leaving air temp effectively strips moisture from the air. However, constantly delivering low temperature air to a space will over cool it, so a modulating hot gas reheat coil is added to reheat the leaving air to a room neutral temperature around 70°.
DOAS and ERU equipment often feature some form of capacity control. Because outside air in cooling seasons can range from 65° to over 100°, these machines must be able to vary the amount of dehumidification they provide. Staged compressors, variable speed compressors, and modulating hot gas bypass are all options.
Space Temperature Control?
No, an outdoor air unit cannot replace a traditional air conditioner as the only device responsible for maintaining temperature in a space. The outside air unit operates constantly, meaning it cannot be turned off when the space is satisfied. Constant operation in a space temperature control design would lead to unacceptable temperature swings. More importantly, the mode of operation for a DOAS/ERU is determined by outside air temperature and humidity, not by space temperature. Space temperature control would become problematic when the space calls for heating, but it is 65° and raining outside. The unit must ignore the space and dehumidify instead, or the space humidity will rise out of control.
Not One or the Other
In order to properly apply a DOAS or ERU, it must be teamed with a comfort cooling device. This decoupling strategy allows designer to meet ventilation requirements while properly controlling space temperature and humidity. Thankfully, at United CoolAir we have solutions for both. Inquire today for more information on our comfort cooling equipment, DOAS, and ERUs.
